This invention pertains to a ferrous metal articles having a protective, adherent, wear resistant coating of metallic oxides and methods of forming a protective, adherent, wear resistant coating of metallic oxides on such articles. The coating desirably has a thickness of from about 6 to about 102 microns (xc2xc mil to about 4 mils) and is formed by exposure of the article to an oxidizing atmosphere, preferably air, preferably during heat treatment of the article. Such articles are useful, inter alia, as torque drive inserts for a friction disk for a multi-disk brake or clutch assembly. Such a friction brake disk assembly has a plurality of axially aligned annular shaped rotor disks splined for axial movement interleaved with annular stator disks which are splined for axial movement along a mating key member or members that are fixedly secured to a torque tube. Each of the stator disks and the rotor disks has a plurality of circumferentially spaced slots along the periphery, with metallic reinforcing drive inserts therein to transfer the load to the disks. The drive inserts are formed of an alloy such as A286 alloy steel having an adherent coating of metal oxides formed by controlled oxidation of the underlying base metal. The inventors have discovered that the material of the drive inserts can have a significant influence on the dynamic stability of a multi-disk brake.
Foley et al. disclose the results of their evaluation of the sliding frictional characteristics of cobalt on cobalt, nickel on nickel and iron on iron under different atmospheric conditions over a range of temperatures using a hemispherically shaped pin in continuous sliding contact at 3.63 m/min on a rotating disk (R. T. Foley, M. B. Peterson, and C. Zapf, Frictional Characteristics of Cobalt, Nickel, and Iron as Influenced by Their Surface Oxide Films, ASLE Transactions 6, 1963, pp. 29-39). Although considerable data is presented that indicates that behavior varies widely among these different metals as the temperature and the atmosphere change, no guidance is given as to the potential behavior of complex systems such as alloyed steels.
Rabinowicz suggests that an oxide film of about 0.01 micron thickness on the base metal is needed to provide effective dry lubrication (E. Rabinowicz, Lubrication of Metal Surfaces by Oxide Films, ASLE Transactions 10, 1967, pp. 400-407). Considerable data is presented that indicates that behavior varies widely among different metals including nickel riding on type 303 stainless steel, type 303 stainless steel riding on nickel, nickel riding on nickel, and type 303 stainless steel riding on itself, as the temperature and the atmosphere change.
A method of forming protective coatings of co-deposited aluminum oxide and titanium oxide on wear surfaces of substrates such as hardened or treated steel or cemented carbides is disclosed in U.S. Pat. No. 4,052,530 to Fonzi. According to this patent, the coating is formed by simultaneously reacting an aluminum halide gas and a titanium halide gas with water on a surface maintained at a temperature of about 900xc2x0 C. to about 1250xc2x0 C. The coating comprises alpha alumina (Al2O3) with about 2% to about 10% hexagonal alpha titanium oxide (Ti2O3) dispersed in the alumina matrix. The process of Fonzi requires use of chemical vapor deposition with attendant special process equipment and costs, and may deleteriously affect the morphology of the substrate ferrous alloy due to use of relatively long times at temperatures.
This invention relates to ferrous alloy articles having a wear resistant coating of metallic oxides that are formed by controlled oxidation of the base alloy, and to a method of forming an adherent, metallic oxides coating on an article comprising a substrate of ferrous alloy of the following general composition expressed in weight present: C 0-0.08, Cr 13.5-16, Ni 24-27, Fe balance. Other alloying elements may be present. The coating has a thickness of from about 6 microns (xc2xc mil) to about 102 microns (4 mils), or from about 13 microns (xc2xd mil) to about 76 microns (3 mils), or from about 25.5 microns (1 mil) to about 51 microns (2 mils) and is formed by exposure of the article to an oxidizing atmosphere, preferably air, preferably during heat treatment of the article.
The present invention also relates to friction braking systems and more particularly to aircraft friction disks with reinforced peripheral slots for use in multiple disk brakes. In brake assemblies which employ a plurality of brake disks alternately splined to the wheel and axle of an aircraft, it is important to provide specially constructed drive means to reinforce peripheral slots in the disks to relieve the severe stressing that otherwise would rapidly deteriorate the periphery of the disks. When disk brakes were constructed of steel, the disks were able to withstand the shearing and compressive forces exerted thereon between the slots and the torque transmitting members because of their physical properties. With the replacement of the steel disks with carbon and/or ceramic composite disks it is important to provide reinforcing drive inserts at the peripheral slots since the composite materials have less durability under this type of loading than steel. The drive inserts transmit the forces to the composite disks, over a larger area reducing the contact stresses which increases the load capability of the composite composite disks.
The present invention preferably utilizes a torque drive insert such as that described in U.S. Pat. No. 4,469,204. Such drive insert in the peripheral slots engages the composite disk and drives on the rotating member or transfers the stress to the stationary member without deleterious effects on the carbon composite materials. The drive insert has a pair of opposed faces which frictional contact the opposite walls of the slots to distribute the load. The drive insert design uses a channel to retain the drive inserts in position within the slot. Such structure eliminates peeling or fraying of the carbon composite heat sink material while also taking up some of the stresses due to misalignment in the wheel and brake. The structure of the drive insert and clip provides for a large bearing area in the carbon composite and minimizes the weight required to obtain the strength to handle the side loads in case of some misalignment. The construction permits the drive insert to float freely in the slot of the carbon disk thereby eliminating the tension loading of the attaching rivets.
The present invention contemplates an iron-chrome alloy article having a protective coating of adherent, wear resistant, metallic oxides and a method of forming a coating of protective, adherent, metallic oxides on an article comprising a substrate of iron-chrome alloy. The coating has a thickness of from about 6 microns (xc2xc mil) to about 102 microns (4 mils), and more preferably is at least about 12 microns (xc2xd mil) in thickness and is formed by exposure of the article to an oxidizing atmosphere, preferably air, preferably during heat treatment of the article. The invention also contemplates most specifically, a multi-disk brake having disks with torque drive inserts formed of oxide-coated A286 alloy steel.
The present invention also contemplates a friction brake disk type assembly with the disks having flat annular surfaces and a plurality of circumferentially spaced slots along the outer periphery of the rotating disks and slots along the inner periphery of a stationery disk. A torque drive insert is positioned within each slot for contact with either the spline of a torque device in the case of the rotating disk or with key members that are fixedly secured to a stationary torque tube. The drive inserts are U-shaped with each leg having a pair of side portions that extend away from the slots along the flat annular surface of the disk. Clips are fixedly secured to the disks and retain the drive inserts within their slots permitting a slight degree of movement because of a clearance space between the clips and the drive inserts. Certain surfaces of the drive insert are subjected to sliding frictional contact with their associated confronting surfaces of the associated torque drive member, e.g. of an aircraft wheel as the loaded wheel rotates. A multi-disk brake provided with drive inserts formed of oxide-coated A286 alloy steel provides increased stability against vibration during a braking event compared to an otherwise identical brake having a brake stack in which all of the drive insert are formed of AMS 5385F cobalt-base alloy which is commercially available as Stellite(trademark) 21 alloy from The Haynes Stellite Co., Kokomo, Ind. or Stoody Deloro Stellite, Inc., St. Louis, Mo.